1. Field of the Invention
The present invention relates to a numerically controlled machine tool capable of machining a non-circular workpiece such as a cam shaft.
2. Description of the Prior Art
Generally, in a numerically controlled grinding machine, the feed of the grinding wheel perpendicular to a spindle axis is controlled by a numerical control apparatus according to profile data and machining cycle data.
The profile data comprises data relating to the amount of movement of the grinding wheel per unit angle rotation of the spindle which defines the reciprocating motion of the grinding wheel along the finished shape of the workpiece. On the other hand, the machining cycle data comprises data relating to a numerical control program to control a machining cycle which includes rapid feed, cutting feed and retracting feed of the grinding wheel.
In the numerically controlled grinding machine tool of the aforementioned type, the ability of the main spindle and the grinding wheel feed axis to accurately follow command data is needed so as to grind the workpiece precisely.
However, it is very difficult to accurately follow command data without a delay occurring. To compensate for such follow delay, the main spindle and the grinding wheel feed axis are numerically controlled according to the execution profile data, which is calculated from the ideal profile data and which is compensated for the follow delay. Since the follow delay is changed depending on the temperature of the feed mechanism, and is changed with a lapse in time, it is very difficult to maintain the accuracy of a finished workpiece within a desired tolerance even when using compensated execution profile data. Accordingly, it is necessary to measure the profile of finished workpiece and to ascertain whether or not the profile error of the finished workpiece is within the prescribed tolerance.
Since prior numerically controlled grinding machines do not have the ability to accomplish the above-mentioned measurement automatically, there is a possibility of producing many substandard products.